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Of the Oxfordian⁄Kimmeridgian

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Of the Oxfordian⁄Kimmeridgian [Palaeontology, Vol. 53, Part 1, 2010, pp. 11–52] THE AULACOSTEPHANIDAE (AMMONOIDEA) OF THE OXFORDIAN⁄ KIMMERIDGIAN BOUNDARY BEDS (UPPER JURASSIC) OF SOUTHERN ENGLAND by JOHN K. WRIGHT Department of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK; e-mail [email protected] Typescript received 30 August 2003; accepted in revised form 14 September 2009 Abstract: Four areas in southern England centred on the Late Oxfordian Pseudocordata Zone into Pseudoyo, Swindon and Westbury (Wiltshire), Bourton (north Dor- Pseudocordata and Evoluta Subzones is confirmed. In the set) and the Dorset coast near Weymouth (south Dorset) Early Kimmeridgian Baylei Zone, however, the evidence have yielded well-preserved late Oxfordian and early Kim- is that the sequence throughout much of southern England meridgian (Upper Jurassic) ammonites in abundance. is incomplete compared with more complete sequences These ammonites belong principally to the aulacostephanid such as that at Staffin in the Isle of Skye, with the pres- genera Ringsteadia and Pictonia, and their microconch ence of only one faunal biohorizon, the densicostata equivalents Microbiplices and Prorasenia. Systematic descrip- horizon. tions of these genera are included herein. Within the zonal and subzonal sequence of the English Oxfordian ⁄ Kimme- Key words: Oxfordian, Kimmeridgian, southern England, ridgian Stage boundary beds, the established subdivision of ammonites, biostratigraphy. T he genera Ringsteadia and Pictonia, and their respective Arkell confined his attention solely to Microbiplices. Sykes microconch equivalents Microbiplices and Prorasenia, and Callomon (1979) subsequently described Upper comprise a major constituent of the late Oxfordian and Oxfordian Amoeboceras, but only one species of Ringste- early Kimmeridgian ammonite faunas of North-West adia, from the Isle of Skye. Matyja et al. (2006) figured Europe. They characterise the Sub-Boreal Province several Ringsteadia and Microbiplices from this locality, (Text-fig. 1), predominantly in England, northern France but, until the present work, there has been no compre- and western Germany and have been used extensively hensive modern study of these genera. Important work as zonal ⁄ subzonal indices throughout the province. on Ringsteadia in the doctoral thesis of Morris (1968) Their range extends northwards well into the Boreal remains unpublished. Province. Pictonia and Prorasenia have never been subject to South and southeastwards in the Sub-Mediterranean detailed modern study by British authors. Of the limited Province, most records of Ringsteadia in the literature work that has been performed, it is worth mentioning have proved to be of a separate, younger, Jura ⁄ Pomera- Buckman (1924b, 1927) and Spath (1935), who nian endemic group homeomorphic with Ringsteadia, but described respectively two new species of Pictonia and in fact having only a very ancestral connection with the two of Prorasenia. Significant faunas of Pictonia and North-West European faunas (Arkell 1956; Wierzbowski Prorasenia from the Isle of Skye were figured by Matyja 1970). et al. (2006). Numerous specimens of Pictonia from Perhaps surprisingly, the British aulacostephanid faunas northern France were figured by Tornquist (1896), but have attracted only a limited amount of research. Ringste- for many years this material lacked a satisfactory modern adia is comparatively well known because of the pioneer- account. This has now been rectified by Hantzpergue ing work of Salfeld (1917) who monographed the (1989). Recent stratigraphical research on the Kimmerid- Ringsteadia faunas of Wiltshire and Dorset. Buckman gian of Normandy has been summarized by Gallois (1919–30) figured several specimens of Ringsteadia. Arkell (2005). Specimens of Pictonia from Greenland have (1935, p. xxxii, 1940, p. lxv) promised a revision of Ring- been described by Birkelund and Callomon (1985) and steadia, but this task was later abandoned as ‘there are from Siberia by Mesezhnikov (1969). Sub-Mediterranean other tasks more urgent’ (Arkell 1947a, p. 352), and specimens recorded as Pictonia lack the regular deep ª The Palaeontological Association doi: 10.1111/j.1475-4983.2009.00916.x 11 12 PALAEONTOLOGY, VOLUME 53 TEXT-FIG. 1. Extent of the Boreal, Sub-Boreal, Sub-Mediterranean and Mediterranean provinces across Europe, with the distribution of land and sea in the latest Oxfordian, and the principal continental localities mentioned in the text (after Ziegler 1982, Cope 1995 and Matyja and Wierzbowski 1995). constrictions and flared ribs characteristic of Boreal and tinue to be useful in order to describe and define the Sub-Boreal forms and should be regarded as a separate range of variation existing within a biospecies, and, in endemic group (Pachypictonia) not directly connected. general, pre-existing specific names are retained here. Specimens which are taken to be the microconch coun- The retention of some of these names is necessary in terpart of Pictonia, are closely similar to the German particular in cases where formal subzones have been Prorasenia of Schindewolf (1925), and this name may named after morphospecies. No new specific names have also be used for the British forms. The aim of the pres- been introduced, however, and where morphotypes have ent paper is to provide the first full descriptions of been found whose morphologies are not covered by uppermost Oxfordian and lowermost Kimmeridgian existing morphospecies, these are described as variants aulacostephanids from southern England based on the of the closest morphospecies. latest understanding of the stratigraphy of these beds Groups of similar morphospecies have then been (Wright 2003). grouped into genera. In a very varied population, differ- ent generic names may have been applied to extreme members of one continuously variable population. An AULACOSTEPHANID TAXONOMY – attempt is made here to allocate all morphospecies of one ‘GENERA’, ‘SUBGENERA’ AND continuously variable biospecific taxon to one genus. The ‘SPECIES’ procedure of Arkell and others of using subgenera to group together assemblages of similar morphospecies is The extreme variability seen in aulacostephanids, partic- not adopted. Such groupings are best referred to by the ularly Ringsteadia, causes significant problems in taxon- name of their most typical morphospecies. When dividing omy. Early workers followed conventional classifications up a single evolving lineage into successive genera, this in terms of typological morphospecies and morphogen- can only be performed on grounds of convenience, per- era having stratigraphical ranges (often not specified or haps historically influenced. even known) – the ‘vertical’ classification. Such classifi- Dimorphism also presents taxonomic problems. Almost cations are purely subjective, arbitrary and hence unnat- all ammonite biospecies occur as dimorphs, the assump- ural. Classification today strives towards a natural tion being that the microconch forms were the males and taxonomy in terms of successions of ‘horizontal’ biospe- the macroconch forms the females (Makowski 1963). The cies (see e.g. Sylvester-Bradley (1956), or Callomon difference in adult size can be enormous – in the case of (1963, 1969, 1981, 1985) for ammonites in particular). Ringsteadia and its microconch counterpart Microbiplices, These are the phyletic transients of an evolving lineage, 30 mm (microconch) and 450 mm (macroconch). In the chronospecies or biospecies of some authors. Some some cases, authors have already decided to allocate one biospecies can be made up of half a dozen or more generic name for the combined dimorphic pair. However, morphospecies. However, morphospecific names con- the changes in characters used to define successive micro- WRIGHT: AULACOSTEPHANIDAE OF OXFORDIAN ⁄ KIMMERIDGIAN BOUNDARY BEDS 13 conch genera do not always occur at the same time as England and the northern part of southern England. In those used to define the successive macroconchs. This southern England from Westbury southwards, the latest being almost certainly the situation with Ringsteadia- Oxfordian is more arenaceous in facies, and the beds are Pictonia and Microbiplices-Prorasenia, separate morpho- allocated to the Sandsfoot Formation succeeded by the generic names for macroconchs and microconchs are Kimmeridge Clay Formation. retained here. In this southern area, sandy and ferruginous beds form convenient marker beds dividing up the clay succession (Text-fig. 3). The Sandsfoot Grit Member and associated AMMONITE STRATIGRAPHY Westbury Ironstone Member separate the Sandsfoot Clay and Ringstead Clay members, clays that are equivalent in This study describes ammonites collected from the Oxfor- age to parts of the Ampthill Clay succession of more dian ⁄ Kimmeridgian boundary beds principally in south- northern areas. The Osmington Mills Ironstone Member ern England. Localities in England referred to in the text marks the highest Sandsfoot Formation, succeeded by are marked in Text-figure 2B–D. The ammonites have the basal Kimeridge Clay Inconstans Bed, sandy, often been collected from predominantly clay facies rocks, phosphatic clays containing abundant, well-preserved much of sedimentation during the latest Oxfordian and ammonites. Kimmeridgian in England being argillaceous in nature. Within the clay members and formations, ammonites Beds are allocated to the Ampthill Clay Formation suc- are often crushed and poorly preserved, but at certain ceeded by the Kimmeridge Clay Formation in northern horizons, the development of micrites
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